Thermal Physics

Question 1

7. Describe an electrical experiment to determine the specific heat capacity c of a liquid.
   Include in your answer (8 marks):
   o A labelled diagram of the arrangement
   o A list of the measurements to be taken
   o An explanation of how the value of c would be determined from your results
   o Possible sources of uncertainty in your measurements and how these could be reduced

Answer 1

Labelled diagram (2 marks):
• Liquid in vessel with electrical heater (submerged) and thermometer.
• Ammeter connected in series between supply and heater AND voltmeter connected across heater
List of measurements (3 marks):
• Mass of liquid
• Initial and final temperature (of the liquid)
• I, V and t values OR energy meter readings OR power and time
Explanation (1 mark):
• E = mcΔθ rearranged to c = E/mΔθ
Uncertainties (max 2 marks) each stated with explanation of remedy:
• Heat losses (makes E or Δθ uncertain) solved by insulating beaker/ use lid
• False temperature reading solved by stirring the liquid
• Temperature continues to rise after heater is switched off solved by measure highest value
• Thermal capacity of vessel solved by take this into account in calculation

Question 2

8. How does a gas exert a pressure on a container wall? (4 marks)

Answer 2

Momentum of the molecule changes when it collides with the wall
Many molecules collide with the walls
Force on the molecule is the rate of change of momentum (Newton’s 2nd Law)
Force on the wall is equal to and opposite to the force on the molecule (Newton’s 3rd Law)
Pressure= (sum of forces (due to all molecules))/(area of the wall)

Question 3

9. Explain in terms of the motion of the molecules of the gas why the volume of the gas must increase if the pressure is to remain constant as the gas is heated (4 marks).

Answer 3

• When gas is heater molecules gain KE/ move faster*
• This would cause more collisions per second with the walls*
• Collisions exert more force / greater change in momentum per collision*
• For constant pressure fewer collisions per second are required
• Constant pressure is achieved by the increase in volume OR with a bigger volume
• So there are fewer collisions per second

(*If the question was: Explain, in terms of the behaviour of molecules, why the pressure of a gas in a container of constant volume increases when the temperature of the gas is increased (2 marks). Then the * points would be the marking points)

Question 4

10. State and explain two different reasons why the actual quantity of energy required to heat the water to 100C in a kettle is greater than the theoretical value (2 marks)

Answer 4

• Some heat/energy used to heat kettle
• Some heat/energy lost to surroundings/air/environment
• Some heat/energy used to boil water before kettle switches off

Question 5

11. State how the observation of Brownian motion lead to conclusions about the nature and properties of the molecules (3 marks)

Answer 5

• Movement of smoke particles caused by randomly moving air molecules
• Smoke particles are continuously moving because the air molecules are continuously moving
• Smoke particles are visible but air molecules are not hence air molecules must be very small
• Small movement of smoke particles is due to the large numbers of air molecules hitting from all sides

Question 6

12. Describe the process of evaporation in terms of the kinetic energy of the molecules and state the three factors which influence the rate of evaporation (5 marks)

Answer 6

• Molecules have a range of KE
• Those with low KE are bound within the liquid, whereas those with high energy can overcome the attraction forces of surface molecules and thereby escape and become vapour molecules
• Some have a medium amount of KE, escape momentarily, but are then pulled back by the attraction forces of surface molecules
• Factors influencing rate of evaporation (1 mark for each): Temperature, surface area, air flow.